1. Field of the Invention
The invention relates generally to a complementary metal-oxide semiconductor (CMOS) image sensor. More particularly, the invention relates to a CMOS image sensor having a Single Frame Capture Mode (SFCM).
A claim of priority is made to Korean Patent applications No. 2004-97418, filed on Nov. 25, 2004, the disclosure of which is hereby incorporated by reference in its entirety.
2. Description of the Related Art
A wide array of consumer and industrial electronic devices incorporate image sensors. These devices include, for example, digital cameras, cellular phones, web cameras, personal digital assistants, and digital video cameras, to name but a few.
The two most popular types of image sensors used in contemporary electronic devices are charge coupled device (CCD) image sensors and complementary metal-oxide semiconductor (CMOS) image sensors. Historically, CCD image sensors have been more popular than CMOS image sensors. However, due to improvements in the CMOS image sensors, an increasing number of devices are incorporating these types of image sensors.
CMOS image sensors currently offer a number of benefits relative to CCD image sensors. For example, CMOS image sensors typically consume less power than CCD image sensors and they also allow other circuit functions such as drivers, clocks, amplifiers, etc., to be included on the same integrated circuit (IC) chip with the CMOS image sensors. In addition, CMOS image sensors can be more highly integrated than CCD image sensors, i.e., a CMOS image sensor can be significantly smaller than a CCD image sensor, and furthermore, unlike CCD image sensors, CMOS image sensors can be manufactured using standard CMOS manufacturing, e.g., equipment used to manufacture processors, memories, and so forth.
Unfortunately, CMOS image sensors still tend to have more noise (e.g., dark current) than CCD image sensors. A part of this noise is caused by on chip circuitry in the CMOS image sensors. Another part of the noise is caused by surface contamination and physical stress introduced by fabrication processes into a floating diffusion layer where electrical charges are. accumulated during operation of the CMOS image sensors.
Another problem with CMOS image sensors is that the response of different pixels in the CMOS image sensors under identical lighting conditions tends to be slightly less uniform than that of CCD image sensors. One reason for the lack of uniform pixel response in CMOS image sensors is that pixels in CMOS image sensors are typically “read out” in a sequence, e.g., line by line, allowing some pixels to be exposed for a longer time than others. The non-uniform exposure time not only leads to non-uniform images, but in some cases. it may even cause some pixels to become “over-exposed”, spilling over charges into adjacent pixels.
Although CCD image sensors also read out pixels in a sequence, CCD image sensors address the problem of non-uniform exposure by capturing images using a “single frame capture mode”. In the single frame capture mode, the CCD image sensor captures a single frame (i.e., an image) by simultaneously converting incident light at all pixels in the CCD image sensor into electrical charges and then converting the electrical charges into voltages representing the image. The simultaneous conversion of incident light into electrical charges may be accomplished, for example, using electronic shuttering.
Unfortunately, conventional CMOS image sensors do not provide a single frame capture mode, as illustrated by the following example shown in FIG. 1.
FIG. 1 is a circuit diagram illustrating a conventional CMOS image sensor. In particular, FIG. 1 shows a pixel sensor in a conventional CMOS image sensor.
Referring to FIG. 1, a pixel sensor of a conventional 4-transistor CMOS image sensor comprises a photodiode region 10 generating electrical charges in response to incident light, and a transfer transistor 12 connected between photodiode region 10 and a floating diffusion layer (or node) 14 and gated by a column selection signal. Transfer transistor 12 transfers electrical charges accumulated in photodiode region 10 to floating diffusion layer 14 in response to the column selection signal.
The pixel sensor further comprises a source follower transistor 20 having a gate connected to node 14 and a source connected to a data output terminal Vout. The voltage of data output terminal Vout varies based on the amount of electrical charges transferred to floating diffusion layer 14.
The pixel sensor still further comprises a reset transistor 16 connected between floating diffusion layer 14 and a power supply voltage VDD. The potential of floating diffusion layer 14 is reset through reset transistor 16.
The pixel sensor still further comprises a column selection transistor 18 connected between the drain of source follower transistor 20 and power supply voltage VDD and gated by the column selection signal. Column selection transistor 18 provides power supply voltage VDD to the drain of source follower transistor 20 in response to the column selection signal.
Where transfer transistor 12 is turned on by the column selection signal, electrical charges accumulated in photodiode region 10 are transferred to floating diffusion layer 14. The gate voltage of source follower transistor 20 varies according to the amount of electrical charges accumulated at floating diffusion layer 14. A voltage of data output terminal Vout caused by the variation of the gate voltage of the source follower transistor 20 is sensed.
The conventional pixel sensor illustrated in FIG. 1 has a contact structure formed on floating diffusion layer 14 to electrically connect the gate of source follower transistor 20 to floating diffusion layer 14. Dark current and noise frequently occur in the pixel sensor due to surface contamination and stress on the floating diffusion layer.
In addition, a 4-transisor pixel sensor is not designed to operate in a single frame capture mode (SFCM). In order to operate in SFCM, all pixel sensors in an image sensor need to simultaneously sense incident light and transfer resulting electrical charges. However, conventional CMOS image sensors using the 4-transistor pixel sensor illustrated in FIG. 1 can not operate in SFCM because transfer transistors 12 in different respective pixel sensors are turned on in sequence. Furthermore, because there is a time delay between when electrical charges stored at each floating diffusion region 14 are output to data output terminal Vout, dark current and noise are produced at floating diffusion layer 14. Accordingly, image quality is degraded.